https://tedebc.ufma.br/jspui/handle/tede/747 Wed, 08 Apr 2026 01:43:11 GMT 2026-04-08T01:43:11Z https://tedebc.ufma.br/jspui/handle/tede/6576 Título: Síntese e Propriedades Estruturais, Vibracionais e Dielétricas das Perovskitas Duplas RE2CuMnO6 (RE = Nd, Eu e Gd) Autor: LIMA, Welber de Jesus Mendes Primeiro orientador: MENEZES, Alan Silva de Abstract: In this work, the structural, vibrational, dielectric and electrical properties of polycrystalline RE2CuMnO6 (RE = Nd, Eu, and Gd) ceramics, synthesised by solid-state reaction, were studied. Characterization by X-ray powder diffraction (PXRD) confirmed the presence of a disordered orthorhombic structure (Pbnm) in all the samples. The microstructure and density were evaluated by scanning electron microscopy (SEM) and the Archimedes method, respectively. Vibrational properties were investigated by Raman and infrared spectroscopy, revealing more intense Jahn-Teller effects in Raman. Fits with the four-parameter semi-quantum model allowed polar phonons to be extracted, and the static dielectric constant (0) extrapolated to the microwave region, which proved to be independent of the ionic radius of RE. Atomistic simulations provided structural and dielectric parameters consistent with the experimental data. Impedance spectroscopy revealed non-Debye behaviour, with a predominance of Maxwell Wagner type interfacial polarisation, responsible for the high values of ε′. Analysis of the Nyquist diagrams showed the electrical response centred on the grains, and the results indicated conduction typical of semiconductors. Temperature-dependent structural analysis showed unprecedented negative thermal expansion in the b parameter. Dielectric and electrical measurements as a function of temperature reinforced the non-Debye character and revealed a negative temperature coefficient of resistance for Eu2CuMnO6 and Gd2CuMnO6. The Nd2CuMnO6 sample showed a positive temperature resistance coefficient, an anomalous behaviour associated with a local metallic character and a possible ferroelectric-paraelectric transition. Finally, the analysis of electrical conductivity and scaling behaviour made it possible to identify the dominant charge transport mechanisms, showing that the electrical conduction mechanism in these materials can change with temperature. Instituição: Universidade Federal do Maranhão Tipo do documento: Tese Fri, 19 Sep 2025 00:00:00 GMT https://tedebc.ufma.br/jspui/handle/tede/6576 2025-09-19T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/6114 Título: Estados eletrônicos, magnetização e corrente persistente em anéis e pontos quânticos controlados por curvatura e rotação Autor: PEREIRA, Luís Fernando Carvalho Primeiro orientador: SILVA, Edilberto Oliveira Abstract: We embark on an investigation of the effects of curvature and inertial forces in a non interacting spinless electron gas confined in mesoscopic systems. In the first case, the thin-layer quantization procedure is considered to study the physical implications due to curvature effects on the spectrum, magnetization, and persistent current of quantum rings and quantum dots both in the presence of external magnetic fields. In such procedure, it arises a geometry induced potential, which depends on both the mean and the Gaussian curvatures. For a ring with a mesoscopic size, the effects of the geometry potential on the energy spectrum are negligible. In magnetization, Aharonov-Bohm (AB) and Haas van Alphen(dHvA) oscillations are observed, while in persistent current only AB-type oscillations are observed. Both physical properties, curvature increases the amplitude of oscillations. On the other hand, at a quantum dot, among the various physical implications due to the curvature of the system, we can mention the absence of the state m = 0. This affects the Fermi energy and, consequently, the magnetization and the persistent current of the model. In the study of magnetization and persistent currents, we found that AB-type oscillations are present, while dHvA-type oscillations are not well defined. In the second case, we analyze how inertial effects can affect several properties of mesoscopic systems. Starting from the Schrödinger equation in a rotating frame, we describe the influence of noninertial effects on the physical properties of quantum rings, 1D and 2D, in the presence of a uniform magnetic field. For 1D quantum ring, we study how electronic states are affected by rotation, and then we investigate how persistent current and magnetization in the ring are influenced by temperature and rotation effects. AB-type oscillations are observed. For 2D quantum ring, we highlight that the potential used to describe the model implies in the appearance of edge states and Landau states (bulk states). It is known in the literature that bulk states do not carry current. As the most relevant case, we report that there is a current carried by the bulk states, which is due to the rotation of the ring. Instituição: Universidade Federal do Maranhão Tipo do documento: Tese Fri, 05 Nov 2021 00:00:00 GMT https://tedebc.ufma.br/jspui/handle/tede/6114 2021-11-05T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/6106 Título: Sólitons autoduais: vórtices e skyrmions em modelos com quebra da simetria de Lorentz Autor: MOTA, Alexsandro Lucena Primeiro orientador: SIFUENTES, Rodolfo Alván Casana Abstract: This research presents an analysis of the existence of topological solitons through the application of the Bogomol’nyi–Prasad–Sommerfield (BPS) formalism in three distinct effective models. These models include: an Abelian Maxwell–Higgs model with a non- minimal covariant derivative that breaks Lorentz symmetry in the CPT-odd sector; a generalized Chern–Simons baby Skyrme model; and a Maxwell–baby Skyrme model with Lorentz-violating terms in the CPT-even sector. In some of these models, adjustments to the original Lagrangian structure were necessary to ensure the proper application of the BPS formalism. The analysis of self-dual solutions in these models revealed solitons with distinct features, including configurations resembling Abrikosov–Nielsen–Olesen vor- tices, Compacton-like structures with Gaussian decay, and others exhibiting power-law behavior. Instituição: Universidade Federal do Maranhão Tipo do documento: Tese Fri, 18 Oct 2019 00:00:00 GMT https://tedebc.ufma.br/jspui/handle/tede/6106 2019-10-18T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/6099 Título: PROPRIEDADES ESTRUTURAIS, VIBRACIONAIS E ELÉTRICAS DAS MANGANITAS ORTORRÔMBICA EuMnO3 e HEXAGONAL ErMnO3 Autor: CORREA, Adriano Primeiro orientador: SANTOS, Clenilton Costa dos Abstract: We investigated the structural, magnetic and vibrational properties of orthorhombic EuMnO3 and hexagonal ErMnO3 manganites using X-Ray Diffraction (XRD), Magnetometry, Raman spectroscopy and Dielectric spectroscopy. The structural and vibrational data obtained respectively via X-ray and Raman diffraction are consistent with the Pnma space group orthorhombic symmetries (D2h 16 -62) and the P63cm space group hexagonal symmetries (C6υ 3 -185)) for the EuMnO3 and ErMnO3 manganites, respectively. X-ray diffraction data as a function of temperature for both manganites show that there are deviations in the monatomic behavior of both lattice parameters and interatomic distances, unprecedented information for both manganites, since structural data as a function of temperature are scarce for these manganites, which allowed us to analyze in more detail the cause of the strong renormalizations of phonon frequencies and evidence of isostructural transitions. In addition, our XRD results for the EuMnO3 manganite give indications of the orbital ordering of the lattice associated with the Jahn-Teller distortion and we also observed, from the structural data, behaviors consistent with the magnetic ordering of the Mn spins in the ac plane, a result that is very interesting since the literature reports difficulty in obtaining the orientation of the spins for EuMnO3. Regarding the ErMnO3 manganite, we observed evidence of the participation of the Mn3+ (site 6c) and Er3+ (site 4b) spin networks, contrary to what is reported for the hexagonal manganites ReMnO3 (Re = Y, Dy, Ho, Tm, Yb and Lu) that indicate only the participation of the spin network of Mn3+ (site 6c) for the magnetic symmetry. In addition, we observed evidence of the occurrence of isostructural transitions involving the Er3+ spin network occurring from the Γ2 Er configuration to the Γ4 Er . The dielectric measurements data for the polycrystalline ErMnO3 manganite allowed us to investigate in more detail the contributions of grains and grain boundaries to the electrical conductivity, important results since the few existing results in the literature are concentrated in monocrystalline samples. In addition, the dielectric measurements showed anomalies in the dielectric constant in the vicinity of the magnetic region, these anomalies, together with magnetometry results, suggest the existence of magnetoelectric coupling for ErMnO3. Finally, we show evidence of a coupling between multi-order parameters and their interaction with the magnetoelectric effect in h-ErMnO3. A high magnetostriction response is evidenced by the Einstein-Grüneisen model. Instituição: Universidade Federal do Maranhão Tipo do documento: Tese Fri, 12 Jul 2024 00:00:00 GMT https://tedebc.ufma.br/jspui/handle/tede/6099 2024-07-12T00:00:00Z